Optical fibers are used as transmission fibers for transmitting light. In particular, the transmission of laser power with high intensity, e.g., to the machining head during material processing (a high power application), can be performed by such fibers. Only a portion of the fibers, e.g., the fiber core, conducts the light with the desired characteristics, whereas the surrounding layers create the external optical conditions and the mechanical stability of the fiber for its specific application.
However, when coupling the light into the fiber and at transition points between fibers, e.g. at plugs or when splicing, leakage radiation escapes from the fiber core into the surrounding layers, e.g., into the cladding. Here, cladding denotes one or several jacket layers of the fiber enclosing the fiber core. From the cladding, which in principle is able to conduct light, the light can be transmitted, e.g., at contact spots or by direct transmission, into the coating of the fiber. The coating provides stability for the fiber and it normally includes one or several plastic layers, e.g., in the form of a buffer directly contacting the fiber and a so-called jacket arranged around the buffer. In the present disclosure, the coating is denoted as sheath of the fiber. This sheath can convey the radiation and as the case may be partially absorb it. However, in particular with high laser power, a high heat build-up of the areas where the radiation escapes, e.g. at a contact spot, can happen. At this locations, the severe heat build-up can lead to a destruction of the fiber. In an extreme case, this can be evenly accompanied with a destruction of the laser source.
Therefore, it is desirable to discharge in controlled manner the leakage radiation from the cladding of the fiber. Therefore, so-called mode strippers are known. These are attached at the beginning or at the end of a transmission fiber or at transition points and they can cause a specific outputting of the leakage radiation.
FIG. 1a shows a fiber which is known from the prior art. The fiber 20 has a fiber core 21 which is surrounded by a fiber jacket 22. The fiber jacket 22 is, in turn, surrounded by a sheath 25 including one or several layers. A mode stripper 26 is formed at the separation layer between the fiber jacket 22 and the sheath 25. In FIG. 1a, the courses of beams of a first light beam S1 and of a second light beam S2 that do not hit the fiber core (“leakage radiation”), and of a light beam S0 that is transported in an intended manner in the core of the fiber are illustrated. The beam S1 has a flat (large) angle of entry with respect to the perpendicular L on the end face of the fiber, i.e., the angle with respect to a fiber axis (radiation with a high numerical aperture NA), so that this beam can escape from the fiber jacket 22 in the region of the mode stripper 26. The beam S2 has a steeper (smaller) angle of entry (radiation with a low NA) so that this beam is not output at the mode stripper 26 and it can be transported over long distances in the fiber.
As shown in FIG. 1b, a beam entering into the fiber 20, in particular into the cladding, in a flatter angle of entry can be steadily reflected and further transported and escape at a contact spot 29 through the bonding material 28 when a sheath is low refractive and poor in absorbing. Here, the contact point 29 connects a plug 27 through a bonding material 28 which directly contacts the fiber jacket 22. Then, the radiation can be absorbed by the bonding material 28 which can lead to a local overheating and, therefore, to a destruction of the fiber 20. Although the problem may be avoided by using an adhesive having a low refraction index and low absorption for the radiation, such adhesives have an insufficient bonding strength so that their application can be disadvantageous.
In FIG. 1b, only the leakage radiation that emerges when coupling in laser light into the fiber is shown. Leakage radiation can also emerge in the fiber at a splicing or at other contact points. As a result, the radiation can enter from the core area into the cladding and further spread or invade into the sheath or bonding and lead to a high local heat build-up of the fiber.
In order to avoid heat build-up in local areas, GB 2 379 279 suggests introducing a layer of lower refractive glass between the cladding and the sheath in order to avoid leakage radiation from entering into the buffer. However, the lower refractive glass prevents the leakage radiation from being discharged from the fiber. As a consequence, the higher or high mode leakage radiation, which rests in the cladding of the fiber, may reduce the beam quality of the transmitted beam. This can have negative influences on applications of the laser. Depending on the angle of entry of the radiation into the fiber jacket, the leakage light may also be transmitted over long distances. The reduced beam quality may be a big problem, in particular with single mode fibers and for applications where a very good beam quality is necessary, e.g., at laser cutting. With specific fibers, e.g., fibers having a thick undoped quartz jacket, substantial leakage radiation may be guided due to the large sectional area.